The present invention relates to an image forming apparatus of electrophotographic type. Further, the present invention relates to an image forming apparatus of electrophotographic type having a corona charger provided with a cleaning member. In an image forming apparatus of electrophotographic type, a photoreceptor is uniformly charged to a predetermined electric potential and, after that, is exposed to light to form an electrostatic latent image on the photoreceptor. Then, the electrostatic latent image is developed with toner and transferred to a recording medium and the transferred image is fixed. In this manner, image formation is conducted.
As a means of uniformly charging the photoreceptor of such an image forming apparatus, a corona discharging device called a scorotron charger is well known. The scorotron charger has been widely used because it can uniformly charge the surface of the photoreceptor with a simple structure. However, the scorotron charger utilizes discharge phenomenon, generating discharge products such as ozone and nitrogen oxides. If such discharge products adhere to the photoreceptor or the charger, image deterioration may be caused, as known in the art. For this, a method for ventilation has been proposed (for example, Japanese Utility Model Publication No. H6-43815). According to this method, an image forming apparatus is provided with an opening for sending air in the longitudinal direction in the back side of the charger and a duct for blowing air to the back side of the charger. By blowing air from one end of the duct to create airflow in the charger through the opening, ozone is exhausted.
However, the proposed method for exhausting ozone has a problem that the successive corona discharge intensifies deterioration of a discharge electrode near the end on the downstream side in the direction of airflow in the charger. As a result, the discharge on the downstream side in the direction of airflow becomes unstable, thus creating a significant defect on a position corresponding to an image. Particularly in case of a halftone image, the difference in density between a right half and a left half of an image maybe intensified.
We studied the cause of this problem and found the following. When a photoreceptor drum is rotated, drum wind created by the rotation of the photoreceptor drum is blown into the charger through an opening on a side of the charger facing the photoreceptor drum and air in the duct is blown into the charger through a ventilation opening on a side of the charger opposite to the side facing the photoreceptor drum. The air is blown into the charger through both the opening on the side of the photoreceptor drum and the opening on the side facing the duct as mentioned above so that ozone stays in the charger at the end in the direction of airflow, thus locally intensifying the adhesion of discharge products.
In particular, under a low temperature and low humidity condition (for example, temperature of 10° C., humidity 15%), since the amount of H2O as carrier of corona discharge is small, the corona discharge is easily unstable, leading to significant adverse effect of the adhesion of discharge products. Therefore, unignorable image defects and density defects are sometimes caused.
Conventionally, a conductive shield composing the charger is provided with a ventilation opening through which air is blown to exhaust ozone to prevent the ozone from staying, thereby preventing adhesion of discharge products on the photoreceptor and the discharge electrode.
However, according to this method, it is required to form the ventilation opening for blowing air into the charger. However, when the ventilation opening is formed in a portion corresponding to the image forming area of the photoreceptor, the position or the shape of the opening may adversely affect the discharge so that the photoreceptor can not be uniformly charged, thus leading to occurrence of image defects such as vertical linear stains on obtained images.
There is also a problem that the adhesion of other matters such as toner or silicone oil onto the discharge electrode also unsettles the discharge, leading to occurrence of an abnormal image. For this, conventionally, a cleaning member is provided which is moved in the tensioning direction while being in contact with the discharge electrode to clean the discharge electrode.
However, as the cleaning member is arranged such that its standby position is on the downstream side in the direction of blown air, the cleaning member in the standby position is contaminated with foreign matters such as scattered toner created during an image forming operation, causing a problem that the cleaning member can not conduct the cleaning operation well or a problem that the contaminated cleaning member adversely contaminates the discharge electrode.